(1) Field of the Invention
The present invention relates to a high hardness rubber composition having an excellent processability and a tire prepared by using this rubber composition. More particularly, the invention relates to a high hardness rubber composition comprising a rubber and, incorporated therein, soft black, a thermosetting resin and a curing agent and a tire formed by using this rubber composition for a structural portion where a high rigidity is required.
(2) Description of the Prior Art
In order to increase the hardness in rubber compositions, large quantities of additives such as carbon black, sulfur and vulcanization accelerators have heretofore been incorporated. For example, it is possible to increase the hardness of a rubber composition, for example, to a level of at least 90 as measured by a JIS spring type hardness tester (hereinafter referred to as "hardness (JIS)"), by incorporating a large quantity of carbon black into a rubber composition. However, this rubber composition can hardly be put for a practical use because it cannot be processed for kneading uniformly in the desired manner, it requires a relatively large power load on the Banbury mixer and it is very difficult to wind the rubber composition onto a roll. Accordingly, in order to increase the hardness while eliminating this disadvantage, the amount incorporated of carbon black is reduced to a low level allowing practical application of the rubber composition and large quantities of sulfur and a vulcanization accelerators are incorporated. However, because of a high sulfur content, blooming of sulfur is caused in an extrudate when this rubber composition is extruded, though the processability at the mixing step is improved to some extent. Generation of blooming results in reduction of the productivity at the manufacture of tires or degradation of properties of products. Furthermore, the hardness (JIS) of this composition reaches its highest point at about 88.
In order to further increase the hardness of this rubber composition, it has been tried to incorporate a thermosetting resin. In this case, softening of the thermosetting resin is promoted by heat generated during the mixing step, and therefore, the processability during the mixing step is improved to some extent and a rubber composition having the hardness (JIS) of at least 90 can be obtained. However, if a thermosetting resin is incorporated into carbon black having an average particle size of 20 to 30 millimicrons (so-called hard black), though the mixing processability is considerably improved as pointed out above, the viscosity of the rubber composition is not reduced, and therefore, the extrusion processability is not improved and the adaptability of an extrudate of this composition to handling is drastically degraded. In case of a rubber composition formed by incorporating carbon black having an average particle size of 20 to 30 millimicrons and a thermosetting resin, the rigidity in the uncured state is extraordinarily higher than the rigidity of a rubber composition free of a thermosetting resin. Furthermore, the self-bonding property inherent of rubber is lost, and the adaptability to such operation as bonding or bending is drastically degraded and the moldability and quality of a tire including an extrudate of this rubber composition are remarkably reduced.